1. Field of the Invention
The present invention relates to an RF module used for propagating a signal in a high frequency band of microwaves, millimeter waves, or the like and a method for arranging through holes in an RF module.
2. Description of the Related Art
Conventionally, as transmission lines for transmitting a high frequency signal in a microwave band, a millimeter wave band, and the like, a strip line, a waveguide, a dielectric waveguide, and the like are known. They are also known as components of a resonator and a filter for high frequency. An example of a module formed by using any of the components for high frequency is an MMIC (Monolithic Microwave Integrated Circuit).
Recently, there is a known structure that a dielectric waveguide line is formed by a layer stacking technique in a circuit board of a multilayer structure. The structure has a plurality of ground conductors stacked while sandwiching dielectrics and through holes having metalized inner face and provided to make the ground conductors conductive, and electromagnetic waves propagate in a region surrounded by the ground conductors and the through holes.
In such a waveguide of a layer stacked type, when the intervals of arranging through holes are too large, electromagnetic waves leak from the intervals between neighboring through holes. It is consequently necessary to set the intervals of providing through holes to be smaller than a certain value. Conventionally, the intervals of providing through holes are generally determined in consideration of signal wavelength and dielectric constant of a dielectric substrate. For example, Japanese Unexamined Patent Application No. Hei 6-53711 discloses a technique of a waveguide in which through holes are provided at intervals each of which is smaller than a cut-off wavelength. Japanese Unexamined Patent Application No. Hei 11-284409 discloses a technique of a waveguide in which through holes are provided at intervals each of which is smaller than the half of a guide wavelength in a travel direction of electromagnetic waves.
As described above, in the conventional waveguide of the layer stacked type, the intervals of providing through holes are determined in consideration of, mainly, a signal wavelength. However, particularly, the relation between the intervals of providing through holes and a conductor loss, a radiation loss, and the like has not been accurately clarified from mathematic viewpoint. The arrangement of through holes considering only the signal wavelength is not always in a true optimum state.